The present invention pertains to combustion and/or gasification of liquid fuels and/or slurries such as spent liquors used in the manufacture of pulp and paper.
The kraft process for pulping is the dominant method for producing pulp and paper. The kraft process, originally developed and patented in 1884, received great impetus for dominance in the 1930s with the introduction of the Tomlinson recovery furnace, where final evaporation and burning of spent liquor, heat recovery and chemical recovery were combined in one unit. In the kraft process white liquor containing the active cooking chemicals, sodium hydroxide (NaOH) and sodium sulfate (Na.sub.2 S) is used for cooking the wood chips to separate cellulose fibers from lignin. Spent cooking chemicals and lignin are washed away from the cellulose fibers with water forming a residual stream called black liquor. The black liquor initially containing about 15% solids is concentrated in a series of multiple-effect evaporators and concentrators to approximately 75% solids. The concentrated liquor is then burned in the Tomlinson recovery furnace to recover the fuel value of the black liquor as steam and the chemicals as an inorganic smelt of sodium carbonate (Na.sub.2 CO.sub.3) and sodium sulfide. The smelt is dissolved in water to form green liquor, which is reacted with quick lime (CaO) to convert Na.sub.2 CO.sub.3 into NaOH and regenerate the original white liquor.
Tomlinson recovery furnaces have been refined and improved over the years, but the basic technology has some major drawbacks. Because the inorganic pulping chemicals are recovered as a smelt in a large quantity in the bottom of the furnace, the industry was plagued by occasional severe recovery furnace explosions. Tomlinson recovery furnaces also present significant environmental challenges. These furnaces produce large amounts of dust that must be reclaimed by electrostatic precipitators. The Tomlinson recovery furnace has a high cost, both initial capital costs and operation and maintenance costs.
Black liquor gasification is a promising new technology which could be a replacement for Tomlinson recovery boilers and yield higher electrical efficiency, with prospective environmental, safety, and capital cost benefits for kraft mills.
High temperature black liquor gasifiers operate at 950.degree. C. (1650.degree. F.) or higher and produce a fuel gas containing primarily H.sub.2, CO, CO.sub.2, and H.sub.2 O and a molten smelt of inorganic chemicals which is immediately quenched and dissolved into green liquor. High temperature gasification, where temperatures exceed the melting point of the smelt and are typically greater than 900.degree. C. (1650.degree. F.), offers the advantages of higher carbon conversion rates and decreased H.sub.2 S generation rates. Black liquor gasifiers can operate at ambient pressure or be pressurized, typically to less than 100 atm. Pressurizing the gasifier allows the equipment size to be reduced, and facilitates feeding the fuel gas to a compression engine or gas turbine.
One of the greatest challenges in gasifying black liquor is in the efficient and reliable introduction and partial combustion of the black liquor in the gasifier. The injector or burner must operate at surrounding temperatures in excess of 950.degree. C., at pressures of 5-25 bar, and in a reducing atmosphere. This has been accomplished in other applications, such as coal and petroleum residue gasification. However, the chemical makeup of black liquor is unique and more problematic than other gasifier feeds. The major inorganic constituents of black liquor, sodium (Na) and sulfur (S) are very corrosive, particularly at the target operating temperatures. The injector/burner must be able to withstand the corrosive and erosive attack of high temperature, high velocity product gas with entrained smelt particles. The burner must also be able to accommodate particulates such as sand particles which can come in with the wood chips. Even with these special challenges, the injector/burner must be able to operate with minimal maintenance to insure that the gasifier can provide an on-stream availability of 98%, which can be achieved from a modern Tomlinson recovery furnaces.
To enable reliable operation with oxygen the burner has to be able to achieve proper atomization and to survive higher flame temperatures produced by oxygen combustion. This means that the droplets should be of optimum size to enable complete carbon burnout, yet not cause over-oxidation of inorganics by exposing them to locally oxidizing regions of the high temperature oxygen flame. There is an on-going discussion among the experts on what exactly is this optimum droplet size. Such information is necessary for the atomizer selection.
Atomization of black liquor for gasification can be achieved using either a pressure or a twin-fluid atomizer. Pressure atomizers, are successfully used for combustion and gasification of coal slurries. However, they have a number of drawbacks, one of the main problems being limited turndown ratio. Pressure atomizers can achieve proper atomization only within limited flow range. To offset this, burner designers use multitude of orifices, usually arranged in a circular pattern within a burner. All orifices are used for atomization during a high firing rate operation. However, for a reduced capacity, some orifices are shut down instead of reducing the fuel flow proportionally. This mode of intermittent operation of some of the orifices causes flame shape variations and requires water cooling of the burner parts exposed to the flame. Another main limitation is erosion and plugging. Pressure atomizers require small orifices for proper atomization. To remedy the turndown ration limitation, the use of multiple orifices reduces average orifice diameter even further. This makes the pressure atomizers susceptible to plugging and/or erosion. A third major limitation is water cooling. Burners utilizing pressure atomizers usually require water cooling which can cause condensation, corrosion, and/or build-up of material on the burner. In turn, this can cause burner malfunction and in extreme cases catastrophic failure.
Another type of nozzle that can be used for black liquor atomization are twin-fluid atomizers. These can be divided into two categories, e.g. internal-mixing, and external-mixing nozzles.
The internal-mixing nozzles are most commonly used with air as the atomizing media for black liquor combustion in recovery boilers. Their maintenance requirements are generally high, due to the plugging of the internal-mixing chamber. Also, they would not be suitable for oxygen enriched combustion due the potential for burner overheating.
External-mixing nozzles can be used for combustion of black liquor with oxygen. However, the process performance including the burner maintenance requirements and overall durability is determined by the ultimate selection of an optimum atomizer and the method of combining the atomizer assembly within a burner system.
Processes and equipment for gasifying cellulosic waste liquor (black liquor) are disclosed and claimed in U.S. Pat. Nos. 5,632,858, 5,352,333 and 5,683,549.
U.S. Pat. Nos. 5,129,333 and 5,363,782 disclose processes and devices for combusting waste fluids containing particulate matter.
U.S. Pat. No. 5,513,583 discloses an apparatus for burning a coal water slurry.
U.S. Pat. Nos. 4,351,645, 4,371,378, 4,364,744, 4,525,175 5,393,220, 5,653,916, 5,785,721 and 5,904,477 disclose processes and devices for partial oxidation or gasification of hydrocarbon materials.
U.S. Pat. No. 4,698,014 discloses a method and apparatus for atomization of viscous liquids or slurries for combustion or gasification.
U.S. Pat. No. 4,857,076 discloses and claims a burner device for production of a synthesis gas consisting essentially of hydrogen and carbon monoxide.
U.S. Pat. No. 5,104,310 discloses a multi orifice oxy-fuel burner.
U.S. Pat. No. 5,617,997 discloses a narrow spray angle atomizer for combustion of liquid fuels such as oil.